As I see, on the first stage I only choose the flat and at the sum up meeting the agreement of deeds of purchase will be signed by all the customers ; so what is the difference between the first agreement and the agreement of deeds of purchase.

At the first stage only agreement should be formed, with it you choose a flat. According to this agreement you are not taking any duties, you just show your interest and open your deposit in the Bank of Georgia. As for the deeds of purchase, it means that you are buying a flat and must pay the money during 20 months.